A known information processing apparatus, which includes a plurality of slots into which, for example, power supply units and other modules may be removably inserted, enables a module stored in one slot to be replaced with the apparatus turned on.
FIGS. 8 and 9 are transparent plan views that transparently illustrate the states of slots of a conventional information processing apparatus 500. In the information processing apparatus 500 in FIGS. 8 and 9, a slot 502a and a slot 502b are formed with a partition wall 504 disposed therebetween.
As illustrated in FIG. 8, a power supply unit 512a is inserted through an opening 513 formed in the rear (on the lower side in FIG. 8) of the information processing apparatus 500 into the slot 502a and a power supply unit 512b is also inserted through another opening 513 formed in the rear into the slot 502b. In the slot 502a, a connector 503 formed at one end of the power supply unit 512a is connected to a connector 512 formed on a back panel 511. In the slot 502b, another connector 503 formed at one end of the power supply unit 512b is connected to another connector 512 formed on the back panel 511.
The power supply unit 512a has a fan 501a that, when driven by the power supply unit 512a inserted into the slot 502a, supplies cooling air. The power supply unit 512b has a fan 501b that, when driven by the power supply unit 512b inserted into the slot 502b, supplies cooling air.
With the power supply unit 512a and power supply unit 512b respectively inserted into the slot 502a and slot 502b, the cooling air supplied by the fan 501a driven by the power supply unit 512a flows from the front into the power supply unit 512a and flows out of the rear, and the cooling air supplied by the fan 501b driven the power supply unit 512b similarly flows from the front into the power supply unit 512b and flows out of the rear, as illustrated in FIG. 8. Heat-generating parts included in the power supply unit 512a and power supply unit 512b are cooled by the cooling airs while the cooling airs are flowing into the power supply unit 512a and power supply unit 512b. 
With the information processing apparatus 500 of this type, at the time of the active replacement of the power supply unit 512a, for example, the power supply unit 512a is pulled out through the opening 513 of the slot 502a is removed while the power supply unit 512b is supplying electric power, as illustrated in FIG. 9. Then, to install a new power supply unit 512c, it is inserted through the opening 513 into the slot 502a. 
Japanese Laid-open Patent Publication No. 10-55661 is an example of related art.
At the time of the active replacement of the power supply unit 512a of the conventional information processing apparatus 500 of this type, however, the cooling air exhausted by the fan 501b, which is used to cool the power supply unit 512b, flows into the slot 502a, from which the power supply unit 512a has been removed.
The exhausted air, which has flowed into the slot 502a, is drawn by the fan 501b; for example, the exhausted air is drawn from the clearance between the back panel 511 and the partition wall 504 into the slot 502b. The exhausted air then flows into the power supply unit 512b. Since the air exhausted by the fan 501b has cooled the power supply unit 512b, the exhausted air is hot. Therefore, the exhausted gas, which has been made hot by cooling the power supply unit 512b, flows again into the power supply unit 512b, reducing the efficiency with which the power supply unit 512b is cooled.